Potential application of notochordal cells for intervertebral disc regeneration: an in vitro assessment

Recent studies suggest that notochordal cells (NCs) might be involved in intervertebral disc homeostasis, a role exploitable to counteract matrix degradation as observed during degeneration. This study aimed to evaluate the potential of NCs to promote matrix production by nucleus pulposus cells (NPCs) and to compare it to the currently proposed addition of bone marrow stromal cells (BMSCs). Using alginate beads, bovine NPCs were exposed for 28 d to porcine NC conditioned medium (NCCM); direct co-culture with porcine NCs or bovine BMSCs; or the combination of BMSCs and NCCM. Effects on cell proliferation, disc matrix production (proteoglycans, collagens) and disc matrix protein expression (aggrecan, collagen 1 and 2, SOX9) were determined and compared to TGFβ stimulation. NCCM strongly promoted NPC proliferation (x 2.2) and matrix production (x 3.9) to levels similar to that with TGFβ, whereas the direct addition of NCs had no effect. Co-culture of NPCs and BMSCs led to proteoglycan synthesis similar to NPCs alone, which was slightly improved by NCCM (x 1.5). Histological analysis confirmed biochemical data. Gene expression of analysed proteins remained stable for all groups and unaffected by medium conditions. NCs could substantially stimulate NPCs through factors secreted into conditioned medium and in levels similar to the addition of BMSCs. This study showed that molecular agents secreted by NCs constitute a promising alternative to the proposed "standard" injection of BMSCs for disc repair: their effects are similar, do not require the injection of a large number of cells and can be further amplified when the factors are identified.